1. Field of the Invention
The present invention relates to a glass substrate and to a glass composition for making a glass substrate. More particularly, the present invention relates to a glass substrate for use as a substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD or of an optical communication device, and to a glass composition for making such a glass substrate.
2. Description of the Prior Art
Conventionally, magnetic disks for use in stationary devices such as desk-top computers and servers typically have substrates made of aluminum alloy, and those for use in portable devices such as notebook computers and mobile computers typically have substrates made of glass. However, aluminum alloy is prone to deformation, and is not hard enough to offer satisfactory surface smoothness on the surfaces of a substrate after polishing. Moreover, when a head makes mechanical contact with a magnetic disk, the magnetic film is liable to exfoliate from the substrate. For these reasons, substrates made of glass, which offer satisfactory surface smoothness and high mechanical strength, are expected to be increasingly used in the future not only in portable devices but also in stationary devices and other home-use information devices.
One known type of glass substrate is those made of chemically strengthened glass, in which the alkali elements present near the surface of the substrate are replaced with other alkali elements in order to produce compression strain and thereby increase mechanical strength. However, chemically strengthened glass requires a complicated ion exchange process, and does not permit reprocessing once ion exchange is complete. This makes it difficult to achieve a high yield rate. Moreover, to permit ion exchange to take place in the glass substrate, alkali ions need to be left easily movable therein. Thus, the alkali ions that are present near the surface of the substrate may move to the surface and exude therefrom during the heating step when the magnetic film is formed, or erode the magnetic film, or degrade the bond strength of the magnetic film.
Another type of glass substrate, known as common glass substrates that are not treated by chemical strengthening, is those made of soda lime. However, soda lime is not mechanically strong or chemically durable enough to be suitable as a material for substrates for information recording. Glass materials used in substrates of liquid crystal panels or the like are so prepared as to have a low or no alkali content so that they have low linear thermal expansion coefficients. This helps maintain thermal stability at high temperatures. However, as a result, these materials have linear thermal expansion coefficients that greatly differ from that of, for example, stainless steel (SUS), of which clamps and spindle motor components are made. This often causes trouble when a recording medium is mounted in a recording device or when information is recorded. Moreover, these materials are not mechanically strong enough to be suitable as a material for substrates for information recording.
Glass substrates are used also as substrates in optical communication devices such as optical filters and optical switches. These devices, however, are occasionally degraded by elution of the alkali content from their glass substrate. Moreover, the higher the density of the film formed on a glass substrate, the more the wavelength shifts ascribable to variations in temperature and humidity can be reduced. There is a limit, however, to the density of a film that can be formed by vapor deposition, which is the method widely used conventionally.
Furthermore, in a case where a glass substrate is used for information recording, when an information recording film is formed on the glass substrate, the pressure, heat, or impact applied to its surface may produce a crack in the glass substrate, leading to a lower product yield.